Email server

Hello,

We are a startup company providing IT based services.
Currently, we are planning to exchange secure documents with our international partners via email.

Our doubts are:
1. Inorder to have a secure Email based data transfer, do we need to setup an Email server inhouse? What are the best email server options available?

2. Once the server is in place, what email clients can we use – i mean Outlook is one option we have in mind. Is there anything better?

We truly appreciate your inputs.
Cheers.